Several forms of heart failure can be treated by ventricular assistance, such as by closed chest compression (an aspect of cardiopulmonary resuscitation), manual heart message, or mechanical ventricular assistance. Closed chest compression, coupled with medication, must be stopped and replaced by some other treatment if effective rhythm and adequate blood flow are not restored expeditiously. Similarly, while manual heart message can be performed for an indefinite period, it is impractical to do so. Manual message also requires a thoracotomy, with its own morbidity, high cost, and potential complications.
Direct mechanical ventricular assistance has been the subject of considerable research for many years. The requirements for highly specialized equipment and major surgery for implantation has limited widespread applicability, especially in emergency situations.
Maintenance of blood circulation by a failing heart can also be provided by removing blood from the ventricles and pumping it back to the aorta. Indirect mechanical ventricular assistance, like direct assistance, requires extensive surgery. It also involves direct contact between the blood and the apparatus. Blood can clot in areas of the apparatus where flow rates are low, and clots can break away and cause a stroke.
Direct mechanical ventricular assist devices have been described in the medical literature and in patents, the following being exemplary:
U.S. Pat. No. 2,826,193 (Vineberg, 1958) PA1 U.S. Pat. No. 3,034,501 (Hewson, 1962) PA1 U.S. Pat. No. 3,233,607 (Bolie, 1966) PA1 U.S. Pat. No. 3,371,662 (Heid et al., 1968) PA1 U.S. Pat. No. 3,455,298 (Anstadt, 1969) PA1 U.S. Pat. No. 3,496,932 (Prisk et al., 1970) PA1 U.S. Pat. No. 4,048,990 (Goetz, 1977) PA1 U.S. Pat. No. 4,690,134 (Snyders, 1987) PA1 U.S. Pat. No. 5,131,905 (Goofers, 1992) PA1 U.S. Pat. No. 5,169,381 (Snyders, 1992) PA1 U.S. Pat. No. 5,256,132 (Snyders, 1993) PA1 U.S. Pat. No. 5,385,528 (Wille, 1995). PA1 An introducer tube and an inserter wire adapted to be passed through the introducer tube and having a distal end attached to the distal edge of the bladder assembly and having a length such that it is adapted to extend out of the proximal end of the introducer tube for manipulation to move the bladder through the incision in the pericardium and into position between the epicardium and the pericardium; PA1 A light-transmitting cable received through the introducer tube for conducting light through the introducer tube to illuminate a portion of the pericardium; PA1 An image receptor and cable received through the introducer tube for transmitting an image of the illuminated portion of the pericardium through the introducer tube to its proximal end. PA1 making an incision in the upper abdomen of a cardiac patient inferior to the xiphoid process and medial to the border of the left coastal arch; PA1 inserting an introducer tube through the abdominal incision; PA1 guiding the introducer tube to a position proximate to the medial aspect of the heart apex; PA1 illuminating a portion of the pericardium proximate to the medial aspect of the heart apex and forming on a monitor an image of said portion; PA1 making an incision in said portion of the pericardium; PA1 providing in collapsed condition a bladder assembly of the type described above; PA1 moving the collapsed bladder assembly through the pericardial incision and guiding it along the heart to a predetermined position, such as by manipulation of an inserter wire; PA1 deploying the collapsed bladder assembly to engage it with the left ventricle; PA1 introducing a fluid material, such as a particulate material entrained in a gas, into the second bladder through the second tube so as to render the second bladder semi-rigid; and PA1 repeatedly pumping a gas under pressure into the first bladder and withdrawing the gas from the first bladder to compress and release the left ventricle.
"First Successful Bridge to Cardiac Transplantation Using Direct Mechanical Ventricular Actuation," J. E. Lowe et al., Ann Thorac Surg 1991; 52:1237-45
"Direct Mechanical Ventricular Actuation: A Review," M. P. Anstadt et al., Resuscitation, 1991; 21:7-23
"The Concept of Direct Mechanical Ventricular Assistance in the Treatment of Left-Ventricular Failure," P. Feindt et al., Thorac. cardiovasc. Surgeon 43 (1995:1-12
Most of the devices proposed heretofore for direct mechanical ventricular actuation are implaced by performing a thoracotomy, opening a window in the pericardium, and placing a cup-like squeezing element over the ventricles. The squeezing element typically has a rigid or semi-rigid outer cup and two chambers, one for each ventricle, formed by panels of an extensible material attached and sealed to the outer cup. The chambers are periodically inflated with a gas under pressure supplied through a tube leading from a mechanical pump to squeeze the ventricles and discharge blood (systole) and then deflated by evacuation by the mechanical pump to draw blood from the atria (diastole).
The squeezing action of the chambers requires that the outer non-extensible cup-like wall member of the squeezing device sustain the reaction loads of the pressure applied to the heart. That means, in turn, that the element must be sized and shaped to fit the heart snugly. Inasmuch as the exact size of the patient's heart is often not known in advance, it is necessary to have a range of sizes of squeezing elements on hand for selection and use after access to the heart has been obtained. While the need to maintain an inventory of squeezing elements and for measuring the heart and selecting an element of the right size is by no means an insurmountable impediment to clinical use of such devices, it is an inconvenience and delays the operative procedure. In an emergency situation, such as heart arrest during surgery, time is critical. The sooner that normal or near normal blood flow can be restored, the lower is the probability of irreversible damage to the patient due to temporary loss of cardiac function.
Because the ventricular portion of the heart is roughly conical, the squeezing action of the squeezing element against the ventricles tends to push the element away from the heart. Thus, it is necessary to hold the element in place. Anstadt et al. (referred to above) provide retention of the element by applying a vacuum within the squeezing element. Snyders (also referred to above) provides retention by suturing the squeezing element to the pericardium.
In addition to the requirement for highly invasive surgery for implanting the device and the need for closing the pericardium window and the thoracotomy wound if the device is to be left in place for a significant period of time, which will almost always be the case, reoperation is required to remove it. Both the operation to implant the assist device and the operation, if required, to remove it place the patient at additional risk beyond the heart condition that called for its use. Even if thoracotomy is not required for removal of the device, as proposed by Goetz (referred to above), surgical implantation of previously known mechanical ventricular assist devices requires relatively complicated surgery that offers little chance for success unless performed by a skilled surgical team in an operating room.
Snyder's '132 and Wilk '528 propose implanting a heart assist device endoscopically, i.e., by inserting the device through an introducer tube that passes through a small incision in the pericardium. The incision is made with the aid of a light source and video imaging equipment for locating the site of the incision and a suitable cutting instrument for making the incision. The assist devices, like those described and shown in other references, are cup-like or cuff-like and thus surround both ventricles and squeeze both ventricles.
The present inventor believes that squeezing both ventricles is inadvisable and may actually be deleterious. That belief is supported by the results of research reported by P. Feindt et al. (cited above).